This invention relates to a cathode ray tube having an envelope comprising a metal rear housing having a wall portion defining an opening and a substantially flat glass faceplate which extends over the opening and is sealed in a vacuum-tight manner to the surface of a flange on the rear housing extending around the opening.
The rear housing, which in the finished cathode-ray tube contains means for producing an electron beam, for example an electron gun, and other components of the tube, is usually referred to as the cone portion of the envelope, although it may not be strictly, or even remotely, conical in the geometric sense. For example, a new type of rear housing which is very relevant to the present invention and which may be referred to as a "flat-can" rear housing has the form of a shallow, rectangular, metal can with a generally flat, slightly dished or ribbed, bottom and an open top surrounded by a flange. An envelope having this type of rear housing is used in so-called flat cathode-ray tubes. To form the envelope a rectangular flat glass faceplate is sealed to the flange of the can.
A cathode ray tube of the type mentioned in the opening paragraph is described in British Patent Application No. 8604321. In order to reduce costs, especially in the case of comparatively large tubes, the metal rear housing of the described tube consists of mild steel for cheapness and ease of forming and the faceplate consists of flat toughened float glass which is significantly cheaper than conventional moulded glass faceplates and which need not be so thick so that it is lighter. Such materials are not ideally matched as regards their coefficients of thermal expansion but by using a compliant pressure bonded seal to bond the faceplate and metal rear housing together and provide a seal therebetween, the effects of the difference in the coefficients of thermal expansion can be accommodated.
The faceplate mounting flange of the tube described in the aforementioned patent application projects inwardly of the opening. The provision of an inwardly-projecting flange together with a compliant pressure bonded seal enables small deflections of the faceplate upon evacuation of the envelope volume to be accommodated generally satisfactorily without the risk of the faceplate peeling away from the flange, and thus avoids the problem encountered with an outwardly-projected flange arrangement such as is described in British patent specification No. 2,133,210 where clamping elements are positioned around the periphery of the faceplate to hold the faceplate firmly against the flange and prevent such peeling.
However, it can become more economical and advantageous particularly with comparatively small sized cathode ray tubes to use plate glass for the faceplate and a thermally-matched metal alloy for the rear housing with a glass frit sealing material bonding the faceplate to the flange of the rear housing.
Glass frit seals are stiffer than pressure bonded seals but, because the faceplate is thermally matched to the metal of the rear housing, provide a convenient form of sealing as it is not required to allow for differences in the coefficients of thermal expansion of the materials of the rear housing and faceplate. Moreover, the dimensions of the seal area need not be so great as those necessary for a compliant pressure bonded seal so that a narrower flange can be used with a consequent reduction in the proportion of the faceplate area used for bonding. Also, higher baking temperatures for outgassing purposes are generally permissible.
This faceplate mounting arrangement has been used for a comparatively small size cathode ray tube having a generally rectangular rear housing defining a rectangular opening with a peripheral, inwardly-projecting, flange to the outer surface of which a rectangular glass faceplate is sealed using glass frit. In such a tube, the size of the seal area becomes more important and so a glass frit seal is advantageous. However, problems have been experienced in that cracks can occasionally appear in the glass of the faceplate at its edges during evacuation of the envelope, or during the subsequent baking of the tube for outgassing purposes as a result of mechanical stresses introduced during evacuation, in the manufacturing process. These cracks have been found to occur more often at corners of the faceplate.
It is one object of the present invention to provide a simple, reliable and effective arrangement for sealingly bonding a flat glass faceplate to a metal rear housing of a cathode ray tube.
It is another object of the present invention to provide a cathode ray tube having a faceplate mounting arrangement capable of withstanding stresses likely to cause damage thereto deriving from manufacturing procedures, such as evacuation and baking, and enabling a glass frit seal to be employed for bonding in a vacuum-tight manner the faceplate to the flange on the rear housing.